The present invention relates to a rear wheel sprocket arrangement for a bicycle, to a drive assembly for a bicycle with such a rear wheel sprocket arrangement, and to a bicycle with such a rear wheel sprocket arrangement or/and with such a drive assembly.
U.S. Pat. No. 3,748,916 A belonging to Morse is considered to be the closest prior art. This patent discloses a rear wheel sprocket arrangement with an extremely large gear range of 500%, which is produced by a total of five (5) coaxially arranged sprockets, namely having the following numbers of teeth: nine (9), fourteen (14), twenty (20), thirty (30), and forty-five (45). The gear range is also referred to as the “gear ratio range” or, in view of the transmission of torque brought about therewith, as the “torque range” or “torque ratio range”. The gear range is the ratio of the number of teeth of the largest sprocket to that of the smallest sprocket.
The individual percentage gear stage steps are very large in this known rear wheel sprocket arrangement, which results in clearly perceptible load jumps for the cyclist using the sprocket arrangement. In the present application, an “individual percentage gear stage step” is understood as meaning the difference in the numbers of teeth of axially directly adjacent sprockets, divided by the number of teeth of the smaller of the adjacent sprockets. The individual percentage gear stage steps of the sprocket arrangement known from U.S. Pat. No. 3,748,916 A—these are in total four (4) gear stage steps in five (5) sprockets—are accordingly 55.6%, 42.9%, 50% and 50% from the smallest sprocket to the largest sprocket. In terms of the arithmetic mean, i.e. the total sum of the individual percentage gear stage steps divided by the number of gear stage steps present, the average percentage gear stage step calculated in this manner is 49.6%, i.e. only slightly less than 50%.
Furthermore, the prior art discloses rear wheel sprocket arrangements which have a larger number of sprockets and, associated therewith, a smaller average percentage gear stage step, in particular because said known rear wheel sprocket arrangements have a considerably smaller gear range despite their larger number of sprockets.
EP 2 048 075 A discloses a rear wheel sprocket arrangement having a total of nine (9) sprockets, the sprockets of which have the following numbers of teeth: eleven (11), thirteen (13), fifteen (15), seventeen (17), twenty (20), twenty-three (23), twenty-six (26), thirty (30), and thirty-four (34). This sprocket arrangement therefore has a gear range of approximately 309% with an average percentage gear stage step of 15.2%.
As a further example of the prior art, reference should be made to a twelve (12) sprocket arrangement which is known from EP 2 022 712 A. This can have, for example, sprockets having the following numbers of teeth: eleven (11), twelve (12), thirteen (13), fourteen (14), fifteen (15), seventeen (17), eighteen (18), nineteen (19), twenty-one (21), twenty-three (23), twenty-five (25), and twenty-seven (27). This sprocket arrangement has a gear range of approximately 245.5% with an average percentage gear stage step of only 8.5%.
These known rear wheel sprocket arrangements no longer always meet modern demands imposed thereon. New demands imposed on sprocket arrangements arise, firstly, from the tendency to reduce the number of front chainrings on the bicycle, optionally even to use just one (1) single chainring, and from the tendency to equip bicycles with electric auxiliary motors. The last-mentioned bicycles are currently generally referred to as pedelecs.
In particular the technical and legal frameworks for the operation of pedelecs, the drives of which are assisted by electric motors, result in demands being imposed on rear wheel sprocket arrangements relating to a large gear range and a comparatively moderate spacing between the individual sprockets in order not to overload the cyclist by torque jumps possibly occurring when changing gear. Furthermore, it should be noted that electric auxiliary motors, also referred to here as “assisting electric motors”, may each generally output its assisting torque only until a predetermined bicycle speed or a predetermined bicycle speed range is reached. At higher speeds, the cyclist is reliant solely on his muscle power.
In the case of what are referred to as pedelecs, there are substantially higher continuous and peak loads acting on the rear wheel sprocket arrangement than in the case of bicycles operated solely by muscle power. The gear changing behavior on the bicycle also changes because of the higher power which is available owing to the assisting motor torque. In the case of conventional bicycles driven solely by muscle power, generally the change from one sprocket of the sprocket arrangement to an axially adjacent sprocket is not made under load or only under a small load. However, in the case of bicycles assisted by an electric motor the change is frequently made, in particular from a sprocket to the next smaller sprocket, under full motor load, i.e. during an acceleration operation. It should be noted here that, in the case of modern pedelecs, the assisting torque output by the electric motor is transmitted, like the muscle power of the cyclist, to the rear wheel of the bicycle via the rear wheel sprocket arrangement. Owing to the high overall torque available in the case of pedelecs from the combination of the torque based on muscle power and the assisting torque of the electric motor, specifically in the low speed range—depending on regulations, assisting the muscle power of the cyclist by an electric motor is usually permitted up to a travel speed of between twenty (20) and thirty (30) kilometers per hour—such high acceleration values can be obtained that undesirable, what are referred to as “multiple gear changing operations” occur, in which a temporally following gear changing operation is initiated before the temporally preceding gear changing operation at the rear wheel sprocket arrangement is finished.
Further changed demands emerge when the number of front chainrings is reduced since the torque transmission range of the bicycle then has to be increasingly provided by the rear wheel sprocket arrangement. This goes so far that, in the case of a single front chainring, the entire torque transmission range which is available depends solely on the configuration of the rear wheel sprocket arrangement.
The reduction in the number of chainrings also may lead to cross-chaining situations of the bicycle chain running between the chainring or the chainrings and the rear wheel sprocket arrangement during operation.